I'm reading Schopen and found his summary quite concise and worth saving:

We know, and have known for some time that the Pāli canon as we have it – and it is generally conceded to be our oldest source – cannot be taken back further than the last quarter of the first centure B.C.E., the date of the Alu-vihāra redaction, the earliest redaction that we can have some knowledge of, and that – for a critical history – it can serve, at the very most, only as a source for Buddhism of this period. But we also know that even this is problematic since, as Malalasekera has pointed out: “ … how far the Tipiṭaka and its commentary reduced to writing at Alu-vihāra resembled them as they have come down to us now, no one can say.” In fact, it is not until the time of the commentaries of Buddhaghosa, Dhammapāla, and others – that is to say, the fifth to sixth centuries C.E. – that we can know anything definite about the actual contents of this canon. We also know that there is no evidence to indicate that a canon existed prior to the Alu-vihāra redaction.

...

We know too that the earliest source we have in an Indian language other than Pāli – and this, according to Norman, is a translation – appears to be the Gāndhārī Dharmapada, the manuscript of which may date to the second century C.E. Of our Sanskirt sources, almost all from Central Asia, probably none is earlier than the fifth century, and the Gilgit Manuscripts, which appear to contain fragments of an Ekottarāgama, are still later. Our Chinese sources do not really begin until the second half of the second century, and it is, in fact, probably not until we arrive at the translations of the Madhyamāgama and the Ekottarāgama by Dharmanandin in the last quarter of the fourth century that we have the first datable sources which allow us to know – however imperfectly – the actual doctrinal content of at least some of the major divisions of the nikāya/āgama literature. It is from this period, then, from the end of the fourth century, that some of the doctrinal content of the Hināyāna canonical literature can finally be definitely dated and actually verified. Not before.

Gregory Schopen, “Two Problems in the History of Indian Buddhism The Layman/Monk Distinction and the Doctrines of the Transference of Merit” in Indian Monastic Buddhism Collected Papers on Textual, Inscriptional and Archaelogical Evidence (New Delhi, India: Motilal Banarsidass Publishers Private Limited, 2010), 23-25.

We know too that the earliest source we have in an Indian language other than Pāli – and this, according to Norman, is a translation – appears to be the Gāndhārī Dharmapada, the manuscript of which may date to the second century C.E.

Minor nitpick, but I believe that the British Library's Kharoṣṭhī fragments are currently considered likelier candidates for "earliest source" in this respect. In "Ancient Buddhist Scrolls from Gandhāra" (1999), Richard Salamon says:

[T]he paleographic and linguistic features of the new manuscripts are broadly attributable to a period ranging from about the early first century A.D. to the middle of the second century A.D., but it is impossible, at least in the current state of our understanding of the Kharoṣṭhī script and the Gāndhārī language, to authoritatively fix them more precisely within this span. The difficulty arises from two problems. The first is the nonstandardization of orthography and morphology, such that variations and changes cannot be fixed to a particular date but rather tend to appear sporadically over long periods. The second problem is that, until now, we have had only one specific specimen, of uncertain date, of a Kharoṣṭhī manuscript of the type that we are concerned with here, namely, the KDhP [=Khotan ("Gāndhārī") Dharmapada] scroll, and nothing to compare it to. The detailed study of the new documents may eventually provide us with the means to refine our understanding, but this will take time.

[...] So, at least for the present, the only way to pin down the date of the documents with more precision is through their historical associations. Here, as we have seen, the references to well-known historical figures in the manuscripts themselves give a firm terminus post quem at about the second decade of the first century A.D. [...] [T]he suggestive pattern of linkages of persons associated with the Apraca lineage and their contemporaries such as Jihonika and the kings of Oḍi, and through them with Kujula Kadphises, the first Kuṣāṇa king of India, makes an early-first-century date, and in particular a date between about A.D. 10 and 30, the most likely one for the composition of the scrolls. Although their paleographic features could be conservatively interpreted as indicating a somewhat later date, perhaps in the late first or early second century, I am inclined to place more weight on the historical data, which, though by no means free of uncertainties, are much more likely to be accurate.

In any case, the references in the manuscripts to Jihonika, Aśpavarman, and possibly other contemporary historical figures prove that the cultural tradition of the texts, if not the scrolls themselves, stems from the Indo-Scythian Buddhist world of the early first century A.D. This point is ultimately more important than the date of the actual manuscripts and is likely to have wide-ranging ramifications for the history of Gandhāran Buddhism [...]

I think the main problem with this summary is that Schopen's conception of historical certainty is unusually strict. It is also a little uneven. I don't think his points of certainty are categorically more certain than the points for which he says there is 'no evidence'. It's a sliding scale of certainty, surely.